You need to start by making a tunnel and you can then extend the undergound tracks in the "sliced" mode and also place stations (also works for roads).It might be a bit difficult to get a tunnel started with the new half height tiles (a tunnel requires a "full" (2 high) tile) but since you're already underground it shouldn't be that much of a problem for you.(you might want to download some nicer stations as the default ones, i know the Londen tube exist and SNFOS (Gauthier) was working on some)All credits to SNFOS

Stations have in their definition whether they are allowed underground or not. Switch to underground mode (not sliced) and the toolbar will be limited to underground stations. If there are none, you'll have to get some add-ons.

Had a look for it and at the moment all the "passenger" platforms are disabled, you can place the cargo ones down (except for the coal one).So to use one of the cargo ones as a passenger station you need either a busstop or a Passenger Building (special construction tools, the crane in the toolbar) as a station extention on ground level to allow people to take the stairs and get off the platform

On the subject as of why the pak snfosstations.pak wont work is that its part of pak 128 already so it wont show up as it is already there(for fun you could add it to pak 64 to show you whats in it, as it will be able to display the stations it just wont line up to the 64 grid)the one you need to go for is subway_station_demi_tile.pakIf you put this in the pak128 folder it should load on opening Simutrans

I didn't even think about that. So essentially I'm just going to connect my above ground bus stops near my town halls to a cargo station below ground. This helps a ton! I'll also look into that pak! Thanks so much!